1. Field of the Invention
This invention concerns a differential mechanism for an automobile, especially a worm gear type differential mechanism with a bias ratio equalization mechanism.
2. Description of the Related Art
A conventional worm gear type differential mechanism is shown in U.S. Pat. No. 4,491,035 and in FIG. 3 of the attached drawing.
As can be seen in FIG. 3, two worm gears (driving gears) 122, 124 are engaged to axle shafts 118, 120 through splines. A thrust element 164 is supported between opposite faces of the gears, the gears being coaxially aligned in a casing 112. The teeth of the worm gears extend in the same direction. When the resistance of the left wheel is the same as the resistance of the right wheel, the rotation transmitted to the casing 112 through a ring gear 117 is transmitted to the axle shafts through worm wheels 136 and the worm gears 122, 124, and the car goes straight. The worm wheels 136 are rotationally mounted on shafts 127 which are attached to the casing 112. In that example, the casing rotates with the worm gears and the worm wheels.
When one of the wheels slips on a muddy or snowy road, creating a difference in the resistances between the wheels, the differential mechanism is operated to effect a relative movement between the shafts 118, 120 through the action of a frictional torque. In the conventional differential mechanism, the worm gears 122, 124 support the thrust element 164 between their opposite faces, and axially contact each other through that element 164. Therefore the thrust forces acting on each of the worm gears by the transmitting torque influence each other in order that the thrust force of one of the worm gears is transmitted to the other worm gear depending upon the direction of the rotation of the casing.
For example, the thrust force F acting on the worm gear 124 (which is operable engaged with the casing 112 through the spacer 180) is a total of the thrust force F1 resulting from engagement between the worm gear 124 and the worm wheel 136 and the thrust force F2 resulting from engagement between the worm gear 122 and the worm wheel 136 (i.e., F=F1+F2). The worm gear 124 is influenced by contact with the worm gear 122 such that F2 is added to the thrust force of the worm gear 124. Therefore the frictional torque of one of the wheels is different from the frictional torque of the other wheel. The frictional torque is added to or subtracted from the driving torque of the wheels in accordance with which of the wheels encounters more resistance. Thus, the value of the frictional torque of one of the wheels is different from the value of the frictional torque of other of the wheels; therefore, when two wheels slip, the torque bias ratio of the one of the wheels is different from the torque bias ratio of the other of the wheels. When the car turns to right or left thereby, the characteristics of the steering are different, and this adversely affects overall driving stability.
In order to solve the above mentioned problem, it has been suggested that the teeth of the worm gears be twisted in mutually opposite directions, so that the thrust forces influence the worm gears in opposite directions, whereby mutual influence of the thrust forces is eliminated. A prior art U.S. Pat. No. 4,491,036 shows this technique and is depicted in FIG. 4 herein. An idler gear (i.e., the central transfer gear) 234 is disposed between the transfer gears 230, 232. The transfer gear 230 engages the transfer gear 232 through the idler gear 234. However, since the mechanism needs idler gears, the mechanism has a shortcoming in that the length of the casing is longer. Further, since the teeth of the worm gears are twisted oppositely, two types of worm gears must be manufactured. Furthermore, since the mechanism needs idler gears (usually three), shafts supporting the idler gears, and the extra hole-drilling operations in the casing, the manufacturing costs are higher.